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MG MGB GT V8 Factory Originals Technical - Austin Ambassador 4 piston Brake calipers available.

For the benefit of those who dont check the classifieds frequently I've listed a pair of Ambassador 4-pots.
John McFeely

Can anyone advise what other modifications would be required along with these calipers?
Alan Boother

I have Ambassador calipers on my car fitted with V8 discs. The bolts need to be modified; I bought two sleeves from Dave Vale at V8 conversions in Farnborough, Kent (the best supplier in the business as far as I'm concerned!). Conversion brake hoses are also needed; I used the ones that came with the Spax kit.
I also have a pair of Four pots from the SD1 that I used as a donor but I think that the castings look a bit bigger and probably wont fit the MG; does anybody know whether they will fit?


The SD1 single line, four pot calipers fit like the Princess/Ambassador ones using sleeves for the bolts or the stepped Triumph GT6 bolts and they centre over solid MGB GT V8 discs. The sleeves I used also came from Dave Vale, as did the metric to imperial hoses and many other parts for the conversion.

The SD1 pistons are slightly larger in diameter compared to the Princess/Ambassador making the calipers slightly larger, they will not fit behind 14 inch RoStyle wheels without (about ¼ inch) spacers. I don’t know about wires but there are no clearance problems with the 15 x 5 ½ inch alloys on my car.

Geoff King


If you no longer need the SD1 calipers let me know, I can put them to a good use.


Dave Brooke


I have the Princess calipers on my car which were dual feed and I have linked them. There is too much pedal travel which is why I am considering changing to the SD1 4 pots with the single feed. I know that people drill the twin feed calipers but am uncertain as to how this is done.


I have princess calipers on my car with a link for the dual feed and I have no problem with travel I think you will find if you put on SD1 calipers with bigger pistons the travel will be more

Mal is correct; the Princess 4 pistons have almost the same surface area as the 2 pistons in the standard MGB caliper so pedal travel should be the same. The SD1 pistons are larger, more fluid is required for the same travel, so more pedal travel to move the fluid. Linked duel feed or single feed makes no difference.

Geoff King


I agree with Mal, you will have more pedal travel with the SD1 calipers. Check the pedal linkage and pivot for wear, plus check the front wheel bearing play.

Also what type of flexible hoses are you using?

Personally I wouldn't drill the caliper, you only need to link the top bleed nipple to the lower fluid inlet with a short copper pipe.


Dave Brooke

If the Princess 4 pistons have almost the same surface area as the 2 pistons in the standard MGB caliper, what's the benefit from changing over?
Dan Masters

The surface area of the pad is larger and you can use the thicker V8 disk or if you change the bridge you can get in a vented disc.

New Princess calipers are not the easiest items to source, but I have tracked down a supplier @ £64.99 each, inc VAT.
Pete Green

I have gone the vented disc route. You can use a 20mm thick Peugeot vented disc and get a steel spacer kit from a brake engineering/rally shop to make the caliper fit perfectly. You will have problems with standard and some aftermarket alloy wheels though.

I have 15x7 Superlites and will have to get a bit machined off the bolt on face to get them to fit (with consultation from the manufacturer I have to add).

Dan, the Peugeot vented disc and spaced Princess caliper is very similar to the RV8 system. In fact, I understand they are nearly identical.

If I could do it over again I would probably spend a little bit more and get Willwood calipers. They're lighter and I'm pretty sure would fit without spacers or wheel mods. Nothing like the perfect vision of hindsight!

David Staines

Standard MGB calipers use two 54mm dia pistons whilst the Princess/Ambassador caliper and RV8 use four 38mm dia pistons. The SD1 in two pot application uses two 57mm dia pistons or in 4 pot applications there are four 41mm dia pistons. Those wishing to do the maths will see the proximity of the surface area between the MGB 54,, standard and the 38mm Princess type.

Add to this the standard single line MGB system uses a 19mm master cyl bore and the later dual circuit uses a 20.6mm bore. Of note is the fact that the Princess/Ambassador used a 20.6mm bore as does the RV8. By way of comparison the SD1 uses 22.2mm bore master cylinders.

The point is that the ratio between the surface area of the master cylinder and the surface area of the slave cylinder (caliper) has to be close to original otherwise you end up with a seriously increased pedal travel as more of the master cylinder strokje is needed to transfer the required volume of fluid for the same piston movement at the caliper. Yes rear wheel cylinder diameter and stroke affects things but the main displacement is at the front.

It follows that it would be unwise to match the single circuit MGB 19mm master cylinder with either of the SD1 calipers, although I know it has been done. The reserve of travel on the master when the brakes are in good condition and all possible free play is removed from the rear brakes is reduced to a level that I personally wouldn't be happy with. In fact I have actually converted my bulkhead to take the late pedal box along with the dual circuit direct servo set up with the 20.6mm master, to match the Princess/Peugeuot brake set up.

I have now a much wider option of alternative master cylinders off the shelf with a larger bore should I wish to go further with the brakes. I decided to do the change of master not just for the safety of dual circuit brakes, but to keep pedal travel at a minimum. One benefit of the smaller master cyl against a bigger one if you can live with the longer travel is that the actual work ratio is increased. This means that for every pound of pressure you input the smaller cylinder with the bigger calipers see increased clamping pressures, or lighter effort for the same braking.

For interests sake the Peugeot discs are the ones from the 604Ti and 505Turbo. They are the same 273mm diamter as the MGB and V8. The RV8 disc is 270mm diameter and a not unsubstantial 25mm thick.

Roger Parker

Thank you for the information on the master cylinder.My '78 is experiencing long pedal stroke after adding 4 pots.Do you know of any bigger bore masters available in the States that will fit my booster?Or,any larger internals that could be used after boring the original? Thanks Mike
Mike Moor

Dear Roger/David,

In Australia what are the Princess/Ambassador calipers known as? Are they readily available, where could I source some from and what is the approx A$ price?

David, you have recommended the Willwood calipers. How are these different to the Princess/Ambassador calipers? Are these also available in Aus?

Graeme Weston

Piston area comparison. One side of one caliper.
MGB. 2x54mm Dia Pistons = 23.89mm Squared per side.
Princess 4x38mm Dia pistons = 22.60mm Squared per side.
SD1 2pot 2x57mm Dia pistons = 25.50mm Squared per side.
SD1 4pot 4x41mm Dia pistons = 26.40mm Squared per side.
Calclated using Rogers information on piston diamaters. Interesting that the princess is actually smaller than the B so anyone experiencing extra pedal travel should look to the condition of the rest of the system.
Leslie Cole

Slight miscalculation ( it was late-ish )in previous thread. It should read as follows:-

Piston area comparison. One side of one caliper.
MGB. 2x54mm Dia Pistons = 22.89 sq mm per side.
Princess 4x38mm Dia pistons = 22.68 sq mm per side.
SD1 2pot 2x57mm Dia pistons = 25.50 sq mm per side.
SD1 4pot 4x41mm Dia pistons = 26.40 sq mm per side.
Calclated using Rogers information on piston diamaters. Interesting that the princess is actually smaller than the B so anyone experiencing extra pedal travel should look to the condition of the rest of the system.
Leslie Cole

Graeme and all,
Wilwood brakes are , as I understand, an aftermarket product used in many hot rods, kit cars, and racers. The castings are alloy and light and the gather good reviews from costomers. They look great in pictures of '38 Ford Coupes too. There have been questiones about Wilwoods' "streetability" reguarding dust seals and warping under pressure. These are old questions and may have been addressed by Wilwood, but I missed it. Is anyone running Wilwoods- for how long -and what for? It would be interesting to hear from someone who has raced thier daily driver on weekends for a few years. Perhaps I should check the '38 Ford section!


Princess calipers are not available in Australia, I brought mine back from the UK. There are conversions advertised here that use Volvo calipers and I have done my own research and found that the mounting holes were 5mm different to the MGB and that the caliper mounting holes had to be machined in order to fit....not ideal if you ask me.

AP is the brand used on the Princess and very similar to the AP four pot used on the SD1. Iknow that Triumph Rover Spares in Adelaide was selling four pot SD1 single line calipers for a hundred dollars each a year ago. Very good price but I can't tell how easy it is to get a spacer kit out here. The SD1 was sold in vented and solid disc form.

I think that Willwood calipers are very light and can be mounted to just about anything. I notice that these are very similar to Outlaw calipers here in Australia. You might be able to find the abovementioned at a hot rod shop somewhere.

I went for the Peugeot discs and Princess calipers for two reasons:

1. To slightly improve the stopping power (arguably not required given the original brakes were not too bad).

2. Get rid of that shocking bloody brake fade. I don't care what anyone says...because my car previously going around some twisty bits in the road for twenty minutes would produce terrible fade.

Hope this helps. If you want some more details give me a direct email.


David Staines

For interests sake, has anyone heard of, seen or done a conversion with a larger diameter than the V8, RV8 or Peugot discs (>273mm) with 15 inch rims.

Graeme Weston

A useful UK based supplier of braking components, who offer bits and pieces to use with Austin Ambassador calipers, is BGC Motorsport Components, 55 Lady's Drove, Emneth, Nr Wisbech, Cambridgeshire, PE14 8DF

Telephone 01945 466690

Fax 01945 467342

E mail : Or


their site has useful drawings of components, and also supply Wilwood kits for the MGB.

I have been using Wilwood calipers since the summer of 1997 on my “factory” B GT V8.

The Wilwood calipers would not fit inside the standard V8 wheels, so I fitted larger Minilite wheels. Initially I used Hawke Brake HB132 Blue 9012 Burnished racing compound pads and Tarox 12 slot disks at about £200 a pair.

This produced excellent braking, but this was expensive, as we used a set of pads and disks on a single Basque rally in 1997 - and the run to Bordeaux and back. Less than 1,000 road miles with little braking and a similar distance rallying in the Pyrenees with lots of hard braking.

I now use Ferodo pads and solid disks, giving noisy, but effective braking at a much lower cost. Despite many efforts, see other threads, I have been unable to eliminate brake squeal when the brakes are cool or used lightly.

I am researching Yellow, Red or Greenstuff pads from EBC, which have a high Kevlar content, and possibly their Sport grooved disks, but would appreciate comments from others who have tried EBC products on V8 MGBs under rally or hard road conditions.

Motorsport World (who have a B V8 Roadster project), appear to have a reasonable we site with a lot of useful information:-

and (of course) there is lots of information regarding brakes at the British Automotive website:-

Safety Fast

Nigel Steward

This thread was discussed between 02/03/2002 and 19/03/2002

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